Thread: Electrical Questions

Tweet

#1--Do I need a relay for my dual horns if they have a 20 amp fuse inline?

#2--Do I need a relay for my 2 driving lights if they are controled by a 30 amp switch?

Thanks to all the gearheads.

Look out for the idiots!

relays are used on most high amp accessories so a smaller switch can be used--------you use a small switch to ground the relay coil with tiny amps to operate the higher load lites or horn---also lessons the high voltage back lash and arcing of switch contacts from the electric field collapsing on the shut off------

Think of a relay as a remote controled switch....that is what it actually is. For the horn circuit, the power to the horns is delivered by the wire going to the horn....so it must safely carry the current to the horn.....in your case, 20 amps or less. However, the wires in the steering column may not be big enough for the 20 amp capacity......so a relay would be proper. The horn button would control the current to the relay and the relay would control the current to the horn. Usually, the horn relay is mounted close to the horn so the larger wire length is minimal. Since horns are on for a very short time, the wire heating up is not an issue

For the driving lights, the same applies. If your switch is rated for more than the current draw of the lights, you dont need a relay but you do need a fuse on the power side of the switch....if the switch shorts out, you want the fuse to blow to protect the rest of the system.

The only thing I would add, is to control the relay circuit by controlling the ground. Especially the horn. Feeding 12 volts positive up a steering column just wouldn't be prudent in my opinion. If that same wire were to short to ground - but control the ground side of the relay circuit, the horn would blow and alert you to the issue, if it was 12 volt positive and going to ground (shorted) you'll need to rely on the fuse to open and protect the circuit. So one day your horn is needed and it won't work. Or worse you've inserted a wrong sized fuse and the wire melts the insulation or worse.

haven't all horn buttons been to ground the relays since back in the hand crank days?????????


And head lights have always used grounds to control high or low beam relays?????????

But we both know that others will wire them using the positive side.

It almost/kinda the same concept of the ampmeter versus the voltmeter. Can you imagine having a 130 AMP charging system with that capability running to a amp gauge on your dash and it goes to ground.

Originally Posted by 34_40

Can you imagine having a 130 AMP charging system with that capability running to a amp gauge on your dash and it goes to ground.

The manufacturers could have switched to a remote shunt to handle the amps and read the millivolt drop across it with a remote indicator. Fuses in the remote line would have protected against shorts. This would have been much more expensive and complicated than the simple voltmeter, so you know the cheaper answer was used by the OEM's. Seems that I recall Ford used something similar to this on the last ammeters that were factory installed. They used a known length section of wire as a shunt.

As for the OP about relays. As has been said, yes on the horns. On the lights, I would, but you could probably get away without it. You could mount the light relay close the the battery under the hood and use a smaller wire to trigger the relay.

Mike

I only tried to point out some of the hazards (or potential hazards) and not knowing the capabilities of the O.P. and many readers, I felt it was prudent.

You are correct about the shunt. I believe Sun also had a shunt arrangement if you went over 60 amps ( it may have been less.. crs!) but the point was a wire capable of supporting better than 100 amps, potentially unfused, inside the cockpit.. just gave me the willies.. so I wanted to mention it.

too back they couldn't fix more things with a tiny red light!!!!!!!!!!!

I've "fixed" more things with the volume knob of the radio that I care to admit!

OK---I am convinced that I need to use a relay on the horns.
I have a set of dual horns that are rated at 20 amps each.
Can I get away with 1 relay or do I need to use 2 relays?

Can someone point me to a SIMPLE wiring diagram for my 2 horns?

Do you not use Google to look for things you need? Googled "Horn Relay Wiring" and this is one of about ten in the first group of pictures - HornWiring-1.jpg

It's amazing what you can find on the net, if you have the correct search words.

Originally Posted by Triker

OK---I am convinced that I need to use a relay on the horns.
I have a set of dual horns that are rated at 20 amps each.
Can I get away with 1 relay or do I need to use 2 relays?

Can someone point me to a SIMPLE wiring diagram for my 2 horns?

Relays, like most other electrical equipment have a rating as well. Most automotive relays are rated 20 to 50 amps. If your horns draw 20 amps each and you have two then you would need a relay rated above 40 amps. I would recommend going to 50amps to have some breathing room. Don't forget the fuse!!!! I looked up relays this morning and found this;

Automotive Relays | WiringProducts

50 amps for 13 bucks. Another 5 bucks for a wiring harness to make a very clean install as well.

Originally Posted by rspears

Do you not use Google to look for things you need? Googled "Horn Relay Wiring" and this is one of about ten in the first group of pictures - Attachment 62812

By the way. The ground side of the relay in this diagram could be used for switching with the horn button as well. Most steering columns (horn buttons) switch ground not the positive. Power would be applied to pin 86 and the horn button would be connected to pin 85. The fuse going to pin 86 would most likely be in the fuse panel where you would get power. If you were taking power directly from the battery, you would still need the fuse shown in that diagram for pin 86. Don't forget that the fuse for pin 30 would still need to be 40+ amps if your two horns draw 20 each and you would need a high current fuse holder as well. Something like this;

MAH1 Series - Fuse Holders Automotive Aftermarket Products from Fuses - Littelfuse